The present invention relates to overheat detectors and, more particularly, to such detectors capable of monitoring for overheating in an extensive area.
Systems for detecting overheating and, therefore, the risk of fire are well known. Some of these systems are resettable and can provide an indication of where in the area being monitored, the overheating has taken place. One such system is described in GB-A-2 149 167. However such systems may be considered to be too expensive for some applications. For example, considerable fire risks are associated with fuel storage tanks of the type which have a floating roof, which moves up and down within an outer wall in order to accommodate a variable amount of fuel within the tank. Inevitably a certain amount of fuel vapour will escape between the outer wall and the roof giving rise to a considerable fire risk on the roof of such tanks. Although the roof of such a tank may cover an extremely large area, it is generally not critical to know precisely where overheating has taken place since the required response will be independent of this information. The ability to reset the detector is also not as critical, as for example, in an aircraft, since the detector on a fuel tank roof can be relatively easily replaced, but it is important that large areas should be capable of being monitored at a reasonable cost.
It has been proposed to provide a non-resettable overheat detector consisting of a twisted wire pair, in which the wires are insulated from one another by a matrix, which melts at increased temperatures to allow the wires to come into contact with each other. Such a detector requires a current to flow along one of the wires so that a short caused by the matrix melting in overheat conditions can be detected. Although this detector is relatively cheap to produce, the requirement for there to be an electric potential itself gives rise to an element of fire risk.
The present invention is directed to solving the technical problem of providing an economical overheat detector that does not require the presence of an electric potential and can therefore be made intrinsically safe.